Inventions described herein relate generally to the field of machine tool operation, and more specifically to methods and systems for anticipating incipient shaping tool deterioration and failure early enough to stop operation of the machine tool, before any tool failure occurs, and thus to avoid damage to the cutting tool, the machine in use, the part being made, its local environment and harm to the operator. Computerized Numerically Controlled machining (so called CNC machining) accounts for a very significant amount of the machining done in modern factories and workshops. CNC machines operate largely under computer control. An operator sets up the machine, including placing a work piece on the machine work table, and providing an appropriate shaping tool in the tool holder. The operator then activates the machine to follow instructions as to shaping location, direction, speed of motion from one location to another, etc., under control of instructions from a computer or processor that has been programmed to fabricate the specific part using the specific machine. The programming is referred to as numerical control (NC).
There are many types of shaping operations and shaping tools. Shaping operations and tools include, but are not limited to: milling, drilling, lathe-turning, sanding, grinding, sawing, and other forms of cutting and shaping. A typical shaping operation is by milling with an end mill. The following discussion, and much of this disclosure uses material cutting in general, and milling more specifically, as an example for discussion purposes. However, it should be understood that all shaping operations and tools suffer from the same general problems and challenges discussed in this background section. And, further, all such shaping operations and machine tools can be improved with inventions disclosed herein, which can be applied thereto. Thus, although the discussions below are generally cast in terms of material cutting, the applicant intends to cover all applicable material shaping tools and machines and operations, as well as cutting operations.
An NC code consists of a block number, a G-code, coordinates, a tool number, and a special function. In other words, the NC code is composed of two big portions; one is geometry information that represents product shape, and the other is motion information that controls the cutting tools in the CNC machine, and it turns into electrical signals to control motors in the CNC machine. There are many cases of CNC machining depending on work piece material (e.g., metal, graphite, plastics, etc.), work piece material hardness, work piece material size, industry (e.g., die and mold, parts), and so on. One of the most widely used combinations of work piece material and cutting tools uses a steel (hardness of HrC 30-35 (KP4M)) for workpiece material and OSG brand cutting tools, sold by OSG USA, Inc., of Irving Tex.
The operation is explained more fully with reference to FIG. 1, which shows schematically a generic CNC machine set up. The machine tool, for instance, a milling machine (not shown) has a work stage 102, which includes a base plate 104, to which a first, X axis stage 106 is coupled, such that the X axis stage can translate along the x axis, relative to the base plate 104. A Y axis stage 108 is coupled to the X axis stage 106 so that the Y axis stage 108 can translate along a y axis relative to the X axis stage 106 and the base plate 104. A work piece 110 is secured to the Y axis stage 108 so that it will not move while it is being shaped. It can be secured by mechanical or vacuum clamps or by some combination of both, or by any other suitable means.
A tool spindle 112 is provided with its own transport apparatus, so that it can be brought near to the work piece 110. The spindle 112 has a socket 114, which carries a shaping or cutting tool 116, for instance a milling tool, such as an end mill. Typically the spindle is arranged to spin the cutting tool 116 at a high rate, so that it can shape the work piece, such as a piece of metal. The spindle 112 thus is provided with a Z axis actuator, so that it can move along the z axis, relative to the work piece 110. The spindle typically also has actuators to actuate it along the x and y axes. Furthermore, the X axis stage is coupled to an X axis servomotor or other actuator 118 and the Y axis stage is coupled to a y axis servomotor or other actuator 120, so that relative motion along each of the x, y and z axes can be obtained between the tool 116 and the work piece 110. The spindle can also typically be actuated to rotate around the x and y axes, so that the tool can be oriented in any desired position relative to the work piece 110.
Other forms of machine tools, such as lathes, can also be CNC. In such a case, the work piece may be set in a chuck to spin around an axis of spin, and a cutting tool is held in a tool holder, which moves the cutting tool either perpendicular to the spin axis, or parallel to it. The height of the cutting tool, relative to the tool axis can be set, or adjusted, or changed under control of the computer.
The shaping of the work piece is accomplished by running the CNC machine to follow a program that dictates the tool's locations, orientations, motion from one location to another, the spindle speed, etc., all or most of which is pre-programmed so that the tool can automatically cut away material from the work piece 110 to form the desired part. The operator does not cause the tool to move by pushing or pulling or guiding it manually in any way. Thus, the operator has no tactile feel for the tool and workpiece interface, as machining progresses.
As the cutting tool cuts, it moves, and it wears down, with some part of it physically wearing away. Eventually, the cutting tool will become so worn that it no longer functions properly. In an ideal situation, the CNC machine operator will anticipate harmful tool wear, and will stop the machine and change the cutting tool before it is so worn that it fails by either damaging the forming part, or damaging itself or the machine. For the most efficient running of the machine, and thus the industrial machine center, which has many machines in its operation, it would be best that the cutting tool be changed at the right time. If it is changed too soon, then some cutting potential of the cutting tool is wasted. If too long a time is waited before changing, some damage can occur to the part, the machine, the operator, etc. Theoretically, a CNC machining center can be operated without any critical issue if operators change a cutting tool when the expected tool life has been reached or when they find any tool wear. However, too early tool changing could be wasteful. Some research has concluded that tools are not used to the useful end of tool life in 62% of applications.
If severe cutting tool failure occurs, CNC plant managers need to discard all of the defective products and rework. Reliability of the CNC plant is affected, and it may lose business. It is important to anticipate the cutting tool failure before any occurs and to act upon that anticipation. There may also be destruction of the CNC machine itself, resulting in loss of its use for a period of time, and the cost of its replacement. Operator or bystander injury, from flying metal or material parts is also a possible risk, as well as fire.
FIG. 2 shows, schematically, in graphical form, tool wear as a function of cutting time. A break-in period 202 is followed by a steady state wear period 204, followed by a rapid wear, or deterioration period 206, followed by a final failure event 208. In both theory and reality, if cutting tool wear continues during the cutting process, tool rupture can occur abruptly at a certain point in time in the deterioration period shown in FIG. 2. As can be seen, the break-in period 202 is characterized by a rapid initial wear rate. The steady state wear period 204 is characterized by a uniform wear rate, and the duration of the steady state period is relatively long as compared to the other periods. During the steady state period B04, the change in tool wear rate can be gradual, up until an accelerating wear rate in the deteriorating period 206, culminating in a failure event 208. A failure event would be some notable physical degradation of the tool, such as fracture, chipping, bending, melting, shearing, extreme wear, etc., such that the tool no longer functions satisfactorily, if at all.
It is desirable to be able to identify or predict the onset of the deteriorating period 206, ideally, before it begins. However, as seen from the graph, the beginning moment of the deteriorating period 206 is difficult to discern, being characterized by a relatively small change in the rate of wear. The operator definitely wants to be able to know to stop the CNC machine from operating before any failure event occurs, so that in fact, no failure event does occur.
The most frequent types of cutting tool failures are: significant tool wear; tool trembling; and tool breakage. Each of those is typically mainly caused by the following reasons, respectively.
Cutting tool wear can occur gradually, when a cutting tool is exposed to one of several conditions, including but not limited to: low spindle speed, high feed rate, or high tool temperature for a long period of time. Variation of work piece material hardness also causes tool wear. Even though it can be estimated by a technique known as Taylor's Equation for Tool Life Expectancy, the cutting tool life expectancy is based on theoretical calculation and, in many practical cases, it is not generally applicable because many cutting tool failures occur before a cutting tool reaches its expected cutting tool life.
There are two principal types of cutting tool wear for a milling machine cutting tool: flank wear and crater wear. For flank wear, the contacted surface of the cutting tool wears out due to friction between the tool and the workpiece and abrasion. As flank wear develops, cutting force increases significantly, affecting the mechanics of cutting. Crater wear can be caused by chips, created during the cutting process on the rake face of the cutting tool. High temperature at the rake face in which the cutting tool meets the chips that are cut from the workpiece most often leads to crater wear.
Cutting tool trembling is not a tool failure, or deterioration, per se, but it leads to deterioration, failure and/or wear. Trembling occurs when a cutting tool is attached to a cutting tool holder improperly, extending too far. Thus, a spindle picks up harmonics from the cutting tool and resonates back out onto the cutting tool cutting surface, where CNC machining is occurring.
As used herein, the term deteriorated, deterioration, or deteriorating refers to a condition of a tool when it may still be physically intact, but its condition is worn or changed so much that it is actually in the early and medium stages of tool deterioration, before tool failure, and cutting tool rupture. Thus, as used below as a term for a label for tool condition, deteriorated or deteriorating means that the tool has shown signs of abnormality significant enough, that it is no longer normal, different from trembling, which would eventually lead to a physical rupture or other disintegration of the tool, such as a fracture or chipping.
There are many reasons for cutting tool rupture and breakage, including but not limited to: continuous tool wear, trembling, inappropriate tool path being followed (either by being automatically driven by the CNC machine, or a human operator). Cutting tool breakage can cause severe damage to the CNC machine itself. Minor cutting tool breakage is likely to result in slight damage to some parts in the CNC machining center, while major breakage could cause catastrophic structural damage to the machine.
There are at least two common, different ways to detect incipient cutting tool deterioration; direct and indirect methods. There are many direct methods. Some methods are visual. An operator may recognize cutting tool deterioration by inspection through a microscope to visually see tool wear. Some methods are tactile. An operator can run a fingernail along the surface of the cutting tool to feel for irregular roughness. The operator can also check (visually or by touch) for an abnormal wavy pattern on a work piece surface. In practice, CNC operators sometimes stop the CNC machine during the machining process to check the cutting tool condition through a microscope or fingernail test. Thus these activities increase production lead time and reduce productivity, at least with respect to pure output per time spent. Thus, these methods are not ideal for the most advanced factory, which aims to have a fully automated manufacturing system without human operators. Further, such methods suffer from inevitable human errors that could lead to critical accidents because the manual methods rely on a human operator's experience and insight for detecting cutting tool failure.
Alternatively, cutting tool condition can be monitored directly by a laser probe system, but such laser inspection also requires stopping the CNC machining process to check the cutting tool condition. The laser probe mainly detects tool rupture, rather than tool wear, so it is also considered a detective control method. Therefore, the direct methods, such as manual and laser probe, are not considered as continuous real-time cutting tool monitoring systems and are not ideal solutions for the most advanced, automated factory.
Indirect methods use different sorts of sensors to detect cutting tool deterioration automatically and to prevent a catastrophic accident during the CNC machining process. For example, systems monitor cutting tool condition by measuring acoustic emission (AE), cutting force, temperature, and vibration. Among those sensor technologies, AE sensor and force/torque sensor technologies are well-known and some manufacturing plants already use force/torque sensor technologies. AE is transient elastic waves within a material, caused by the rapid release of localized stress energy. (Thus, although the word acoustic is part of its name, it is not a technique that monitors audible sounds.) An event source, such as a crack formation, or elastic deformation, releases elastic energy into the material, which then propagates as an elastic wave. The phenomenon of radiation of acoustic (elastic) waves in solids that occurs when a material undergoes irreversible changes in its internal structure. Causes of plastic deformation include but are not limited to: aging, temperature gradients or external mechanical forces. Several research papers show the effectiveness of AE signal analysis on monitoring cutting tool condition. There are however, limitations. For instance, the AE signal generated from a work piece varies depending on work piece material. Out of range AE signals are hard to separate. Rapid stress-releasing events generate a spectrum of stress waves starting at 0 Hz, and typically falling off at several MHz. Importantly, AE can detect only the ongoing cutting tool failure as it happens (rather than anticipating failure before it happens) because of the nature of the AE signal. It is also critical that the AE signals have a broad frequency range, so that an AE sensor generates too large an amount of information to process practically, as to do so would require huge processing power, which leads to inefficiency. (Acoustic emissions can be detected in frequency ranges under 1 kHz, and have been reported at frequencies up to 100 MHz, but most of the released energy is within the 1 kHz to 1 MHz range.)
Similarly, the force/torque sensor technology is widely known, but it also has a critical limitation. Even if the sensor accurately identifies cutting tool failure during the cutting process, it is highly likely to be too late to address the severe cutting tool deterioration. In other words, the force/torque sensor generally detects the late stage of cutting tool wear or the cutting tool failure at the moment when severe tool breakage is just beginning. Thus, even though the force/torque sensor detects the cutting tool failure and stops the CNC machine, it is very difficult to prevent severe cutting tool failures and, occasionally, machine parts failures.
The same deficiency (late identification) also afflicts most other sensor technologies, such as vibration and temperature sensors. Thus, the use of current sensor technologies can be considered as a detective control only, which identifies severe cutting tool failures in the late stage of cutting tool deterioration, rather than a preventive control that detects or anticipates incipient cutting tool failure in the early and medium stages of cutting tool deterioration and prevents severe cutting tool failure proactively. In addition, the known sensor technologies are relatively expensive for small and medium-sized manufacturing companies, so the cutting tool failure prevention system using an AE sensor or a force/torque sensor is unlikely to be an affordable solution for many manufacturing plants.
Another method that experienced CNC machine operators use to identify cutting tool failure is by listening for audible, abnormal cutting sounds during the CNC machining process. If they hear a significantly abnormal sound, or sound that they recognize from past experience indicates that tool deterioration is happening, or soon to happen, then they stop the machine. This method also has many limitations. It is relatively difficult to do, and thus is not available for novice, or relatively low skilled CNC operators. It is particularly difficult to identify the early and medium stages of tool deterioration, and thus it is more likely that audible methods lead only to detecting tool deterioration when the deterioration becomes severe, tool breakage happens, or when defective final products are already being produced. Also it is almost impossible for CNC operators to detect all the possible tool failures because of inevitable human errors. Furthermore, in typically noisy CNC machine centers, it is difficult to hear the sounds of any one machine tool, especially if the operator is using ear covers or other sound dampening equipment to prevent hearing loss. Additionally, one operator cannot monitor more than one or at most several machines, personally. Thus, audible detection by human operator is not an automatable method, and is labor and human operator intensive.
Another limitation of human operator monitoring and evaluating of the audible sound of operating CNC machines is that acquiring this ability takes significant experience, not only in general, but with each different machine (for instance a lathe, a milling machine, and end mill, a side mill, a saw, a drill press, etc.) and even with each differently shaped part or different work piece material. Thus, use of such a method cannot be had immediately upon beginning the manufacture of a specifically designed part for the first time.
Because the current sensor technologies are ineffective and inefficient at preventing cutting tool failures, small and medium-sized manufacturing companies, desire more advanced but affordable technology that can prevent the cutting tool failure in the early and medium stages of cutting tool wear automatically.
Thus, there is a need to be able to anticipate, or sense or detect tool deterioration before it becomes too late to stop the machine before the tool fails, or breaks. A further need is to be able to anticipate or sense such incipient deteriorating tool condition using technologies that are modest in cost and complexity, so that they can be used by the many small machine facilities around the world. Another need is to be able to anticipate or detect incipient tool deterioration automatically, without the need of an operator to check each machine for deterioration. Yet another need is to be able to anticipate or detect incipient tool deterioration in the dirty, noisy, busy conditions of large and small CNC machine centers. Still another need is to be able to anticipate or sense incipient tool deterioration for all or most of the many different types of CNC machines, tools, and work pieces, alone, and in combination. Still another need is to be able to anticipate or identify incipient tool deterioration without stopping the machine to inspect the tool or the work piece, either directly or indirectly.
Thus, an object of an invention hereof is to be able to anticipate or sense or detect incipient tool deterioration before it becomes too late to stop the machine so that the tool does not break. A further object is to anticipate or sense such incipient tool deterioration using technologies that are modest in cost and complexity. Another object of inventions hereof is to anticipate or detect incipient tool deterioration automatically, without the need of an operator to check each machine for failure. Yet another object is to anticipate or detect incipient tool deterioration in the dirty, noisy, busy conditions of large and small CNC machine centers. Still another object is to anticipate or sense incipient tool deterioration for all or most of the many different types of CNC machines, tools, and work pieces, alone, and in combination. Still another object of inventions hereof is to anticipate or identify incipient tool deterioration without stopping the machine to inspect the tool or the work piece, either directly or indirectly.
These and other objects and aspects of inventions disclosed herein will be better understood with reference to the Figures of the Drawing, of which: